1. Field of the Invention
The present invention relates to the technical field of a power system transformer control, in particular to a transformer control device and method based on three-dimensional zone diagram policy.
2. The Prior Arts
With the rapid development of power industry, continuous grid expansion and dramatic power load growth enable people to have more requirements for power quality and reliability. The voltage of a power system is an important parameter to determine power quality. Large voltage fluctuation not only can affect efficiency and worklife of electrical equipment, but also can lead to voltage collapse and even serious accidents caused by blackout. Transformers are important power equipment in a power system. In a power system, the total number of transformers is more than that of generators, and the total capacity of transformers is much higher than the total capacity of generators and the total capacity of electric motors, so power loss of transformers accounts for total power loss of the whole power system of about 30%. Therefore, operating conditions of transformers have direct effects on economical operation of power systems, and low power factors even affect economical operation of wires and transformers, which are extremely unfavorable for energy saving and improvement of power supply capability of power equipment. In order to ensure stable and economical operation of the grid, operating conditions of transformers must also be optimized when power quality is improved, power factor is increased, and energy loss is reduced.
Common plane-zone diagram methods mainly include nine-zone diagram method and improvements thereof and five-zone diagram method. The nine-zone diagram method is a basic method for general control of substation voltage and is a typical two-parameter control policy including voltage and power factor. In the nine-zone diagram method, a voltage-reactive plane is divided into 9 zones according to constant voltage and upper and lower limits of the power factor, as shown in FIG. 1. In FIG. 1, the ordinate represents the voltage U on the low voltage side of the transformer, the abscissa represents the power factor cos φ on the high voltage side of the transformer, UH represents upper voltage limit, UL represents lower voltage limit, cos φL represents the lower limit of the power factor, and cos φH represents the upper limit of the power factor. The ordinate and the abscissa are respectively divided into three sections with the parameters. For the ordinate, if a point is higher than UH, then the voltage is out of the upper voltage limit; if a point is lower than UL, then the voltage is out of the lower voltage limit. For the abscissa, if a point is higher than cos φH then the power factor is out of the upper limit; if a point is lower than cos φL, then the power factor is out of the lower limit. The nine-zone diagram aims to control the voltage and the power factor within the upper limit and the lower limit, namely within the range of the nine zones. If the voltage and the power factor do not meet the requirements at the same time, the voltage should take precedence over the power factor.
In addition to the traditional nine-zone diagram, many researchers study on the traditional nine-zone diagram for some improvements. At present, common improved zone methods include eleven-zone diagram method, fifteen-zone diagram method, seventeen-zone diagram method, etc.
The plane of the eleven-zone diagram method is shown in FIG. 2. The eleven-zone diagram method is almost similar to the nine-zone diagram method, but one sub-zone is respectively added to the original Zone 2 and Zone 6 to prevent equipment vibration and frequent operation. According to the control policy, when the operating condition is in Zone 8, the control policy of the traditional nine-zone diagram method is used, i.e. transformer taps should be adjusted first to increase the voltage. If the transformer taps are adjusted to the top position, the capacitors should be switched on. Then, the operating condition is in Zone 9 or Zone 20, so adjustment is not necessary. If the transformer taps are not in the top position, the operating condition will enter Zone 20 after the transformer taps are adjusted to a higher position. Then, the transformer taps are adjusted to a much higher position, and the capacitors are switched off, so that the operating condition enters Zone 2 first before entering Zone 9. Therefore, the control policy can effectively prevent equipment vibration and frequent operation.
The plane-zone diagram of the fifteen-zone diagram method is shown in FIG. 3. The difference between the fifteen-zone diagram and the traditional nine-zone diagram is that two dotted lines are added between the upper voltage limit and the lower voltage limit. The dotted lines in FIG. 3 represent critical parameters, the zone between the dotted lines represents the voltage acceptable zone, and the zones on two sides of the dotted lines represent voltage dead zones. The critical parameters are set as required. Compared with the traditional nine-zone diagram, the fifteen-zone diagram has six more control zones and has the advantages of more precise control and less equipment operations.
The plane-zone diagram of the seventeen-zone diagram method is shown in FIG. 4. In the seventeen-zone diagram method, the control policy adopts automatic timing and is self-adaptive to the wiring of the system. According to requirements of the system, five control policies can be adopted, including consideration of voltage only, consideration of reactive power only, voltage precedence, reactive power precedence and overall consideration. Different zones need different control policies. In general, voltage precedence should be considered first for a substation, and then reactive power.
The five-zone diagram adopts the voltage reactive power control (VQC) principle and has totally different controlled objects from the traditional nine-zone diagram. The principle is estimated based on operation effects, is determined according to quality distances and is used for VQC devices, as shown in FIG. 5.
According to characters of operating actions to VQC devices, the operating actions include:                No action        Higher transformer position        Lower transformer position        Switching capacitors on        Switching capacitors off        
The optimal operating action is selected according to given controlled objects and locked constraints as the final operation, so the control concept forms, in which operating actions to VQC devices are directly utilized as controlled objects. Therefore, compared with the nine-zone diagram, the five-zone diagram has more precise and more effective control policy.
A vector diagram can be obtained as shown in FIG. 6 by vectoring current operating conditions on the U-Q plane with the five operations above.
In the text above, the two-parameter control policy of the voltage-power factor (reactive power) is considered. However, in the actual operation of a substation, another important parameter is not considered, namely operating conditions of transformers. Therefore, some problems may occur, for example:
(1) A substation has multiple transformers. When both the voltage and the power factor are satisfied, operators cannot determine the operating mode to optimize the operating mode of the substation.
(2) The operating mode of the transformers is determined according to the economy load parameter of the transformers only without considering the voltage and the power factor. Although transformer load factor is equal to the economy load parameter, the power factor is very low, so transformer loss is not the minimum.